1. Field of the Invention
The present invention relates to a method of manufacturing a nonvolatile memory device.
2. Background Art
With the remarkable proliferation of information communications equipment, particularly, personal small items such as portable terminals, various kinds of semiconductor devices of memory devices and logic devices forming the equipment are requested to have higher performance of higher integration, higher speed, lower power consumption, and the like. Especially, a nonvolatile memory is considered to be indispensable for the ubiquitous age. Even in exhaustion or trouble of a power supply, or disconnection between a server and a network due to some failure, important information can be saved and protected by the nonvolatile memory. Further, the recent portable equipment is designed to suppress the power consumption as much as possible by allowing an unnecessary circuit block to stand by. If a nonvolatile memory serving as a high-speed work memory and a large-capacity storage memory can be realized, wasted power consumption and memory can be eliminated. In addition, the “instant-on” function that enables instantaneous activation when the power is turned on can be exerted if the high-speed and large-capacity storage nonvolatile memory can be realized.
As the nonvolatile memory, a flash memory using a semiconductor material and a ferroelectric random access memory (FERAM) using a ferroelectric material, and the like may be cited. However, the flash memory has a disadvantage with a slow writing speed in the order of microseconds. On the other hand, in the FERAM, the rewritable times is 1012 to 1014. The problems that the rewritable times of the FERAM are not sufficient for replacement of an SRAM or DRAM with the FERAM, and microfabrication of the ferroelectric layer is difficult are pointed out.
As a nonvolatile memory device that does not have these disadvantages, a nonvolatile memory device called an MRAM (magnetic random access memory) attracts attention. Among the MRAMs, an MRAM using a TMR (tunnel magnetoresistance) effect attracts a lot of attention because of the recent improvement in characteristics of the TMR material. The TMR-type MRAM has a simple structure and is easy to be scaled, and has many rewritable times because recording is performed by the rotation of magnetic moment. Furthermore, a very high speed is expected with respect to the access time, and it is said that the MRAM has already been operable at 100 MHz.
Now, in the MRAM, in order to stably hold the recorded information, it is necessary that the recording layer for recording information has a certain coercive force. On the other hand, in order to rewrite the recorded information, a certain degree of current should be flown in the bit-line. However, with the miniaturization of the MRAM, the bit-line becomes thinner, and it is becoming difficult to flow a sufficient current. Accordingly, as a configuration capable of magnetization reversal with a smaller current, a spin injection magnetoresistance-effect device using magnetization reversal by spin injection attracts attention (e.g., see JP-A-2003-017782). Here, the magnetization reversal by spin injection is a phenomenon that electrons spin-polarized through a magnetic material are injected into another magnetic material, and thereby, magnetization reversal occurs in the other magnetic material. In the spin injection magnetoresistance-effect device, compared to the MRAM, the device structure can be made simpler. Further, since the magnetization reversal by spin injection is utilized, compared to the MRAM in which magnetization reversal is performed by an external magnetic field, the device has advantages that the writing current is not increased even when the miniaturization of the device is advanced and that the cell area can be reduced.
A nonvolatile memory device including a spin injection magnetization reversal writing system has a structure (referred to as “MTJ structure”) in which a tunnel insulator film in thickness of about 1 nm is sandwiched between two magnetic material layers (a recording layer also called a magnetization free layer and a magnetization reference layer called a magnetization fixed layer). Further, for practical application of the nonvolatile memory device having the MTJ structure, a process technology of patterning the magnetic material layer without occurrence of damage, short circuit, or current leak is necessary. However, since a transition metal magnetic material is often used for the materials forming the magnetic material layer, in a metal etching technology using a halogen gas used for the etching process of the widely used silicon semiconductor device, it may be impossible to easily etch the magnetic material layer. In addition, it is difficult to etch the magnetic material layer without occurrence of short circuit or current leak in the tunnel insulator film as extremely thin as about 1 nm.
Accordingly, study on a reactive ion etching (RIE) method using a non-halogen gas is conducted. However, since the RIE method has a physical etching element, it is highly likely that the etched material is reattached to the side wall of the tunnel insulator film and short circuit and current leak occur. Further, it is difficult to avoid the occurrence of damage in the MTJ structure of the nonvolatile memory device, and that may be a major factor causing variations in properties.
For patterning of the magnetic material layer, the ion beam etching method has been also used in related art. In the method, although the magnetic material layer is less damaged, the reattachment of the etched material is the most serious issue. Especially, since the tunnel insulator film is as extremely thin as about 1 nm, there is a problem that occurrence of short circuit and current leak due to the reattachment of the etched material at a certain rate is unavoidable.
As measures for solving the above described problems, a technology, at patterning of the MTJ structure, of deteriorating the conductivity by quitting the etching of the magnetic material layer formed on the tunnel insulator film (referred to as “upper magnetic material layer”) in the middle and oxidizing a part of the upper magnetic material layer with a portion of the upper magnetic material layer left is known from JP-T-2003-505873 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application), JP-A-2003-078185, and JP-T-2006-520105. In these technologies, the etching surface does not reach the tunnel insulator film, and thus, no etched material is reattached to the side wall of the tunnel insulator film and occurrence of the problems of short circuit and current leak can be avoided.
Alternatively, as the nonvolatile memory device, a resistance change memory device including a phase change RAM (PRAM) is proposed. The nonvolatile memory device has a structure in which a resistance change layer that functions as a memory part is arranged between upper and lower electrodes, has a simple memory structure, and can easily be miniaturized. The phase change memory device as a kind of the resistance change memory device is a nonvolatile memory device that operates as a memory device using difference of several digits of electric resistance between the amorphous state and the crystal state of the phase change material forming the resistance change layer (for example, see JP-A-2007-134676). Further, there is a nonvolatile memory device that stores data using colossal electro-resistance effect (CER effect) of the material forming the resistance change layer has as a kind of the resistance change memory device (for example, see JP-A-2003-068983). Alternatively, there is a nonvolatile memory device in which the resistance change layer is formed by an ionic conductor containing a metal as a kind of the resistance change memory device (for example, see JP-A-2005-166976 and JP-A-2005-197634). Furthermore, a PMC (Programmable metallization Cell) is known as a kind of the resistance change memory device (for example, see JP-A-2005-322942).
In the resistance change memory device, in patterning of the resistance change layer, it is highly likely that the etched material is reattached to the side wall of the resistance change layer and short circuit and current leak occur.